A fuel cell is an electrochemical device which reacts hydrogen, a fuel source, and oxygen, which is usually derived from the ambient air, to produce electricity, water, and heat. The basic process is highly efficient, and fuel cells fueled by pure hydrogen are substantially pollution free. Further, since fuel cells can be assembled into modules of various sizes, power systems have been developed to produce a wide range of electrical power outputs. As a result of these attributes, fuel cell power systems hold a great deal of promise as an environmentally friendly and viable source of electricity for a great number of applications.
One of a number of known fuel cell technologies is the proton exchange membrane (PEM) fuel cell. The fundamental electrochemical process under which PEM fuel cells operate is well understood and known in the art. A typical single PEM fuel cell produces a useful voltage of about 0.45 to about 0.70 Volts DC, although most fuel cells are operated at about 0.60 Volts DC in order to extract the greatest efficiency from same. To achieve a useful voltage, typically a number of individual fuel cells are electrically combined or coupled in series. In one common configuration, a number of individual fuel cells are electrically coupled in series in the form of a fuel cell stack. In a stack configuration, the anode of one fuel cell is electrically coupled to the cathode of another fuel cell in order to connect the two fuel cells in series. Any number of fuel cells can be similarly stacked together to achieve the desired output voltage and current. Typically, these individual fuel cells are separated by an electrically conductive bipolar separator plate. Further, the individual fuel cells are placed between two end plates and a substantial compressive force is applied to same in order to effectively seal same, and to achieve an operatively effective ohmic electrical connection between the respective fuel cells.
In addition to the relatively low operating temperature PEM fuel cells noted, above Solid Oxide Fuel Cells (SOFC) have been developed. A SOFC is a fuel cell which generates electricity directly from a chemical reaction, yet unlike PEM fuel cells, an SOFC is typically composed of solid ceramic materials. The selection of the materials employed in such prior art SOFC devices is dictated, to a large degree, by the high operating temperatures (600-800 degrees C.) which are experienced by such devices. In view of the extremely high operating temperatures which are needed to render the ceramic based electrolyte ionically active, SOFC devices do not require the use of an expensive catalyst (platinum), which is the case with PEM fuel cells as discussed, above. As a result of these high operating temperatures, assorted fuels can be employed with a SOFC which could not normally be used in a PEM fuel cell. Therefore, SOFC devices can employ fuels such as methane, propane, butane, fermentation gas, gasified biomass, etc. In a typical SOFC device, a ceramic based electrolyte formed of a material such as zirconium oxide is sandwiched between a porous, electrically conductive cathode layer, and a porous, electrically conductive anode layer. These cathode and anode layers are typically ceramic gas diffusion layers that are selected for their structural rigidity and high temperature tolerance. The chosen electrolyte must be impervious to air (oxygen) and must be electrically insulating so that the electrons resulting from the oxidation reaction on the anode side are forced to travel through an external circuit before reaching the cathode side of the SOFC. In a typical SOFC device a metal or electrically conductive interconnect electrically couples the respective cells in a serial arrangement. If a ceramic interconnect is employed it must be extremely stable because it is exposed to both the oxidizing and reducing side of the SOFC at high temperatures.
As should be understood from the discussion above, the cost of fabricating such SOFC devices is significant. Further, to render such devices operational, a rather significant and sophisticated balance of plant arrangement must be employed to controllably heat the SOFC device up to an operational temperature, and then maintain the device within an acceptable temperature range. In contrast, PEM fuel cells do not need the extremely high temperatures employed in SOFC devices in order to render the electrolyte (typically Nafion) ionically active. Further these high temperatures have dictated the use of heat tolerant ceramic materials for the anode and cathode. The cost of fabricating these components is significant. In typical PEM fuel cell devices, the designers of same have continually strived to employ lower cost components, and simplify any balance of plant requirements in order to reduce the cost of same and make the cost per watt of power generated more acceptable for discreet market applications.
While traditional PEM fuel cell stacks have operated with some degree of success, a number of shortcomings continue to distract from their usefulness. First among these shortcomings is the high cost of manufacture for the individual components of a traditional stack design. Chief among these high cost components is the bipolar plate which is employed with same. In order to save costs, many manufacturers of fuel cell stacks have attempted to combine a number of functions into the bipolar plate. A modern bipolar plate is a precisely fabricated component that performs a number of functions including fuel management, cooling, electrical conduction, and gas separation. The result of this combination of functions is that performance in many areas must be sacrificed in order to save costs. Examples of such an arrangement are seen in U.S. Pat. Nos. 5,252,410, and 5,863,671, the teachings of which are incorporated by reference herein.
Another primary cost or factor which impacts a traditional fuel cell stack is that attributed to the force compression needed to make such devices operational. In order to achieve an operationally effective electrical conductivity between a proton exchange membrane, a gas diffusion layer, and/or a bipolar plate, a great deal of force must be applied between the end plates of the traditional stack. Typically, these compression forces are in excess of 100 pounds per square inch. To achieve this level of compressive force, costly, heavy, and complex components are often required. The application of this force typically compresses same components within a stack, for those components which are porous, this same force may reduce the porosity of same. Yet another shortcoming attributable to the traditional fuel cell stack design or arrangement is heat management. Because a fuel cell generates heat while generating electricity, excess heat is often created and accumulates in the center and other locations within the stack. A number of sophisticated technologies and designs have been developed to manage these hotspots, but the result has been higher manufacturing costs and greater complexity for a resulting fuel cell stack system.
One proposed solution to the problems associated with the cost and complexity of prior art force compression arrangements was disclosed in U.S. Pat. No. 6,716,549, the teachings of which are hereby incorporated by reference. This proposed solution involved coating a surface of a traditional, rigid, carbon gas diffusion layer with a metal such that the resulting metalized gas diffusion layer retained its porosity. The resulting metalized surface of the carbon gas diffusion layer forms an ohmic contact with an adjacent metal current collector without the need for the high compression forces that would typically be required without the metal layer. While this solution addresses some of the issues associated with force compression, it still requires the use of a rigid carbon, gas diffusion layer, which has become increasingly costly and difficult to procure. Thus, a long felt need remains for a lower cost, and higher performing alternative to the prior art high force compression proton exchange membrane fuel cell stacks which are now disclosed in the art.
The prior art is replete with numerous examples of other prior art fuel cell devices that attempt to address these and other issues. The Office's attention is directed to U.S. Pat. Nos. 5,470,671; 5,482,792; and U.S. Application Publication No. 2006/0134498; the teachings of which are incorporated by reference herein.
A proton exchange membrane fuel cell stack and an associated proton exchange membrane fuel cell stack module which avoids the shortcomings attendant with the prior art devices and practices utilized heretofore is the subject matter of the present application.